Alda Merini was born in Milan on March 21, 1931 to a family of modest means. Her father was a clerk for a national insurance company (Assicurazioni Generali di Venezia) and her mother was a housewife. Alda had an older sister, Anna, and a younger brother, Ezio. Her father encouraged Alda's interest in reading and writing and published a little booklet of her poems when she was ten. He also had a teacher come to the house to give Alda piano lessons. Alda completed elementary school with excellent grades, but at the end of the war her father had no source of income so Alda was sent to the vocational school Laura Solera Mangazza for three years to study stenography. She then applied for admission to the high school Liceo Manzoni, but was not accepted due to an insufficient score on the Italian entry test.

Alda's career as a poet started at very early age. At fifteen she began devoting herself to writing poems. A year later, in 1947, through Silvana Rovelli - cousin of the poet Ada Negri - some of these poems were sent to the influential literary critic Giacinto Spagnoletti. Thanks to Spagnoletti, Alda began frequenting the period's most remarkable literary circle in Milan, attended by many eminent poets and critics, among them Pier Paolo Pasolini, Maria Corti, Luciano Erba and Giorgio Manganelli, with whom Alda began a tormented love affair. Also in 1947, she showed her first symptoms of mental illness and spent a month at the Villa Turro clinic in Milan. In 1949 the affair with Manganelli ended. In 1950 Alda became romantically involved with Salvatore Quasimodo; the relationship lasted until 1953.

When in 1950 Spagnoletti published Alda's poems "Il Gobbo" and "Luce" in his anthology Poesia italiana contemporanea 1909-1949, her poetry captivated illustrious readers such as Quasimodo, Pasolini and Montale. Montale suggested to Vanni Scheiwiller that he include a group of Alda's lyrics in his anthology Poetesse del Novecento, which was published in 1951.

In 1953, Alda married Ettore Carniti, the owner of a bakery and pastry shop in Milan, and published her first book, La presenza di Orfeo. In 1955, Alda published three books: Paura di Dio (Scheiwiller), a collection that included all of Alda's poems from 1947 to 1953, Nozze romane (Swartz), and La pazza della porta accanto (Bompiani). That same year, Alda's first daughter, Emanuela (Manuela), was born. The baby's pediatrician, Pietro di Pasquale, became Alda's unrequited love interest. To him, she dedicated her next book Tu sei Pietro (Scheiwiller, 1961). In 1958 her second daughter, Flavia, was born.

After the publication of Tu sei Pietro, Alda stopped writing for two decades, due to her deteriorating mental health. In 1965, committed by her husband, she entered the Paolo Pini asylum in Milan, where she remained until 1972, although she did spend some brief periods with her family. During this period, she gave birth to two more daughters, Barbara and Simona.

In 1979 Alda emerged from her illness and began writing again, permeating her poems with her dramatic and painful experience in mental institutions. But after so many years away from Milan's literary circle, she could not find a publisher. In 1982, thanks to the intervention of Maria Corti, the journal "Il cavallo di Troia" published thirty of her lyrics, which were the embryo of her next book. After the death of her husband in 1983, Alda began a correspondence with the doctor Michele Pierri, thirty years her senior. She married him the same year, moving to his home in Taranto. Here she had another bout of serious mental illness and for a short time entered a local mental institution. Vanni Scheiwiller again supported her, bringing out the book Terra Santa in 1984.

Alda returned to Milan in 1986, where she was treated by the psychiatrist Marcella Rizzo, to whom she dedicated several lyrics. Finally achieving some serenity, Alda was able to live outside of mental institutions in her own apartment on the Naviglio. The next twenty years were creatively fecund and Alda published at least a book of poems or prose annually. In 1986 Scheiwiller published her autobiography L'altra verità: diario di una diversa; followed by Fogli bianchi (1987) Testamento (1988), Delirio amoroso (1989), and Il tormento delle figure (1990). In 1991 Alda wrote two new books, Le parole di Alda Merini and Vuoto d'amore, edited by Maria Corti.

Between 1992 and 1996, she published several more books: Ipotenusa d'amore; La Palude di Manganelli, o il monarca del re (dedicated to the poet she once loved, who had died a year earlier); La presenza di Orfeo; Titano amori intorno; Reato di vita; Ballate non pagate; and La pazza della porta accanto. In 1993 Alda received the prestigious Librex-Guggenheim Eugenio Montale Prize, thus becoming an acclaimed and revered poet. In 1996 she won another prestigious prize, the Premio Viareggio, and was nominated by the French Academy for the Nobel Prize.

From the mid-1990s, Alda had heterogeneous artistic collaborations and enjoyed popularity: quite a few of her lyrics were musicalized; some of her books were complemented with drawings; engraved covers enhanced some of her work; many of her poems were recited by famous actors with her on stage; and film directors made documentaries about her life. Intense in these years was the publication of aphorisms by the smaller publisher Pulcinoelefante. In 1997 Alda won the Premio Procida-Elsa Morante. Two new collection of lyrics, Superba è la Notte and L'anima innamorata appeared in 2000. In 2001 she was nominated again for the Nobel Prize.

Alda continued writing until the end of her life: Folle, folle, folle d'amore per te and Magnificat. Un incontro con Maria were published in 2003 and Poema della croce in 2004; in 2005 she produced Uomini miei, and Sono nata il ventuno a primavera: Diario e nuove poesie. La vera novella came out in 2006 and Lettere a Dottor G in 2008. Alda Merini died on November 1, 2009 of cancer.